[GuardCat]

Суперпростой пример.

<div> Наведи сюда </div>
  <script>
    var div = document.querySelector("div"), id, placeholder = div.innerHTML;
    div.onmouseover = makeSomething;
    div.onmouseout = dontMakeSomething;
    function makeSomething() {
      id = setInterval(function(){div.innerHTML = Math.random()}, 200)
    }
    function dontMakeSomething() {
      div.innerHTML = placeholder;
      if(id) clearInterval(id);
    }
  </script>